Origin of the Effect

Benzene, the model pi system has no permanent dipole moment, as the contributions of the weakly polar carbon-hydrogen bonds cancel due to molecular symmetry. However, the electron-rich pi system above and below the benzene ring hosts a partial negative charge. In order to counterbalance this sandwiching negative charge, a positive charge is associated within the plane in which all benzene atoms lie. An (electric) quadrupole (a pair of dipoles, which do not cancel each other) results. The negatively charged pi system can then interact favorably with positively charged ions.

Influences on the Strength of the Cation-pi Interaction

The cation-pi interaction is comparable in strength to hydrogen bonding and can in some cases be a decisive intermolecular force. Several criteria influence the strength of the bonding: the nature of the cation, the subsitutents on the pi system, as well as the solvent.

Substituents on pi system

The electronic properties of the substituents on the pi system also have an influence on the strength of the attraction. Electron withdrawing groups (e. g. Cyano -CN) decrease the amount of negative charge in the pi system and thus weaken the interaction. On the contrary, electron donating substituents (e.g. amino –NH2) increase the charge separation of the quadrupole and strengthen the cation-pi binding. This relationship is illustrated quantitatively in the margin for several substituents.

Influence of the solvent

Additionally, the nature of the solvent also determines the relative strength of the bonding. Most data on cation-pi interaction is acquired in the gas phase, as the attraction is most pronounced in that case. Any intermediating solvent molecule will attenuate the effect, which is why it becomes less pronounced with increasing solvent polarity.

Cation-pi Interaction in Nature

Nature’s building blocks consist of aromatic moieties, too. Amino acid side chains of tryptophane and tyrosine or the DNA bases are capable of binding to cationic species (not only metal ions, but also charged amino acid side chains, ...). Therefore, cation-pi interactions can play an important role in stabilizing the three dimensional structure of a protein. A very impressive example is given by the nicotinamide acetylcholine receptor whose molecular recognition mechanism of its substrateacetylcholine (a positively charged molecule) nearly entirely bases on cation-pi interaction.

Anion-pi interaction

In many respects, anion-pi interaction is opposite to cation-pi interaction, although the underlying principles are identical. Significantly less examples are known to date. In order to attract a negative charge, the charge distribution of the pi system has to be reversed. This is achieved by placing several strong electron withdrawing substituents along the pi system (e. g. hexafluorobenzene). The anion-pi effect is advantageously exploited in chemical sensors for specific anions.